1. Field of Invention
The present invention relates to an AC plug for the supply of AC power supplied from a power outlet to prongs by an AC cord when inserting the prongs into the power outlet and to an electrical apparatus provided with that AC plug and AC cord.
2. Description of the Related Art
An input part of an electrical apparatus powered by a commercial AC power supply has been provided with a noise filter for suppressing noise flowing in from the commercial power line and noise conversely flowing out from the apparatus.
FIG. 8 is a view of an electrical apparatus provided with an AC plug. This electrical apparatus 100 is comprised of an apparatus body 101 provided with an internal circuit 102 and a noise filter 103 at a power input side of the circuit. The noise filter 103 is connected to an AC cord 110 supplying power to the apparatus body and has an AC plug 120 attached to its front. As an AC plug 120, normally a general one comprised of a plastic plug body provided with a pair of metal pieces (prongs) for connecting the two lines of the AC cord 110 is used.
When the AC plug 120 is inserted into a not shown power outlet supplying commercial AC power, power is supplied to the internal circuit 102 of the apparatus body 101 through the AC cord 110.
The noise filter 103 for removing noise at this time includes a noise-suppression capacitor C electrically connected between the lines of the AC cord 110 for preventing leakage of noise generated at the internal circuit 102 of the apparatus body 101 to the outside and further for preventing noise from the outside from being input to the internal circuit 102.
The noise-suppression capacitor C has a large capacitance of for example 1 μF from the need for suppressing noise due to electromagnetic interference (EMI). In this case, a considerably large amount of charge can build up.
Therefore, when pulling out the AC plug 120 from the power outlet, the charge built up at the noise-suppression capacitor C may be applied across the two prongs exposed at the front end of the AC plug 120. To reduce the discharge, a resistance (hereinafter “discharge resistor”) is electrically connected between the two lines of the AC cord 110 inside the noise filter at the apparatus body 101 side.
The resistance value of the discharge resistor R must match with the capacitance between the lines of the AC cord 110 including that of the capacitor C for noise suppression. That is, electrical apparatuses using capacitors between power lines are regulated in many countries. For example, Japan's Electrical Appliance and Material Safety Law requires that the voltage across terminals of an AC plug (voltage across prongs) be no more than 45V one second after the power plug of the electrical apparatus is pulled out from the power outlet. There are similar regulations in other countries as well. In particular, in Europe and other regions where the voltage level of the commercial power supply is high, tougher regulations restrict electrical apparatuses so as to enable more reliable discharge of the prongs. Further, leakage of noise is also strictly governed in many countries.
However, the discharge resistor leads to an increase of the power consumption since it continues to be supplied with current while the AC plug 120 is plugged into the power outlet. In particular, in the standby state, the power consumption of the apparatus itself falls, but the amount of power consumed by the discharge resistor R is constant, so the ratio becomes sufficiently large.
To overcome this disadvantage, it is known to provide a switch inside the noise filter of the apparatus body to prevent wasted current flowing to the discharge resistor (for example, see Japanese Unexamined Patent Publication (Kokai) No. 11-177369). Further, a circuit configuration has been proposed for detecting on a circuit basis whether commercial power is being supplied from the AC plug and cutting off the discharge resistor when the commercial power is being supplied (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2001-095261).
However, in the technology described in Japanese Unexamined Patent Publication (Kokai) No. 11-177369, if operation of the switch is forgotten, the charge will not be released. Further, the power consumption is liable to increase.
In the technology described in Japanese Unexamined Patent Publication (Kokai) No. 2001-095261, if the switch is not operated due to a malfunction of the circuit, the prongs will not be discharged. Further, if the switch is left on, the power consumption will increase, so a malfunction-free detection circuit is required. There would then be the disadvantage of that much higher a cost.
Further, if the discharge resistor is in the apparatus body far in distance from the location of electric shock, that is, the AC plug, even if discharge is started, a residual charge will remain for a while across the prongs at the front end of the AC plug. Therefore, the above-mentioned electrical apparatuses have been insufficiently designed from the viewpoint of reliable discharge.